<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <script src="../vue.min.js"></script>
    <title>Document</title>
</head>
<body>
    <divid="app">
     <p>First name: <input type="text" v-model="firstName"></p>
    <p>Last name: <input type="text" v-model="lastName"></p>
     <p>fullname:{{fullname}}</p>
    <button @click="change">改变fullname</button>
        </div>
</body>
<script>
        vm = new Vue({
        el: '#app', 
        data: { 
            km: 0, 
            m:0
        },
    methods: {
        kmtom: function(val) {
            this.m = this.km * 1000;
            },
             mtokm: function(val) {

             }
            },
            watch: {
                 km: 'kmtom',
                 m: 'mtokm'
                }
            })
     
</script>
</html>